This invention relates to photo-detectors and more particularly to an infrared image detector.
It is well established in the infrared photodetector field that an infrared detector can be built utilizing epitaxial lead salt thin film technology. Single crystal semiconductor films of lead chalcogenides can be epitaxially grown on heated alkali halide and alkaline earth halide substrates by vacuum evaporation. The lead chalcogenides used include the sulfides, selenides, tellurides and mixtures thereof. The substrates are single crystals of infrared transparent alkali halides and alkaline earth halides. Examples include barium fluoride, strontium fluoride, calcium fluoride, lithium fluoride, sodium chloride, potassium chloride, etc. Next, a metal semiconductor contact such as lead or indium is deposited on the surface of an epitaxial layer of the lead chalcogenide thereby creating a non-ohmic Schottky barrier at the point of contact resulting in an infrared sensitive diode. A detailed description of infrared sensitive diode concepts, suitable materials and methods of fabrication are disclosed more fully in U.S. Pat. No. 4,406,050, entitled "Method for Fabricating Lead Halide Sensitized Infrared Photodiodes", which was issued to the present inventor on Sept. 27, 1983, and is herein incorporated by reference.
It is also well established in the prior art that a multi-color, self-filtering detector can be built by depositing successive epilayers of lead chalcogenide on the base substrate. Each layer is normal to the surface of the substrate and contiguous in directions parallel to the surface of the substrate. Each layer is photoresponsive to infrared radiation based upon its chemical composition and is also a photon filter for the succeeding layers, i.e. each layer is used as a detection layer and as a filter layer. The response yielded by the detection portion of each layer is measured as a voltage versus the frequency of an infrared wave band. The wave band of each layer yielding a measurable voltage is determined by the chemical composition of the layer. Thus, the resulting infrared detector can be built to receive radiation in predetermined wave bands. Typical and illustrative of this technology is U.S. Pat. No. 4,323,911 which was issued to Campbell.
In FIG. 1, there is shown a cross-sectional view of a prior art infrared detector having N detector elements. A first semiconductor layer made of N separate materials 11A, 11B, . . . 11N and a second semiconductor layer made of N separate materials 12A,12B . . . ,12N is deposited on a base substrate 10. Layer 12 forms a detection layer made of N distinct materials defining N optical areas. Each optical area has different optical and radiation absorption properties due to differences in chemical composition. Each optical area will therefore respond to infrared radiation over a different wave band of the infrared spectrum. Accordingly, optical area 12A is fitted with a semiconductor metal contact 13A and a lead wire 14A. Metal contact 13A is a non-ohmic contact. Each adjacent contiguous detection layer 12B, . . . ,12N is similarly fitted with non-ohmic contacts 13B, . . . ,13N and lead wires 14B, . . . ,14N. Each optical area must also be equipped with an ohmic contact or ground wire 15A,15B . . . ,15N which is in turn attached to an appropriate lead wire 16A,16B . . . ,16N.
The aforementioned construction of prior art infrared image detectors poses several technical disadvantages. First, the surface of detection layer 12 must be insulated against short circuits when electrical connections are made to the metal contacts 13A,13B . . . ,13N thereby adding complexity and cost to the fabrication process. Second, the detection layer 12 is optically and electrically active in the areas 17 not covered by the metal contacts thereby causing optical cross-talk between detector elements.